1. Field of the Invention
The present invention relates to a process for producing high density SiC sintered bodies having high strength.
2. Related Art Statement
In the case of a process for producing high density SiC sintered bodies by primarily firing shaped bodies made of .beta.-SiC added with B, C and HIP treating them, a high primary firing temperature makes SiC grains take in pores and undergo abnormal grain growth over a few hundred microns during a final firing stage. Consequently, their physical properties are deteriorated, and it becomes difficult to increase density because the pores taken inside the abnormally grown grains cannot be removed by the HIP treatment. When the primary firing temperature is lowered to avoid the abnormal grain growth, the density is not fully increased, and great densification cannot be realized even by the HIP treatment due to the presence of open pores. That is, in order to increase the density by the HIP treatment, it was necessary to strictly control the primary firi,ng temperature range to a narrow width so that the open pore might be diminished and that remaining closed pores might be present in grain boundaries only.
From the above standpoint of view, Japanese Patent Application Laid-open No. 60-255,672 discloses a process for obtaining SiC sintered bodies having a high density of not less than 99.3% and a high strength of not less than 70 kg/mm.sup.2 by strictly controlling the additive composition range of B and C in SiC added with B and C, the primary firing conditions, and the HIP treating conditions to B=0.2 to 0.5 wt %, C=0.5 to 8 wt %, 1,950 to 2,150.degree. C., 1,850.degree. to 1,980.degree. C., and not less than 100 atms, respectively.
Further, Japanese Patent Application Laid-open No. 52-6,716 discloses a process for producing SiC sintered bodies having pores in grain boundaries and a composite structure composed of a planar .alpha.-SiC and finer .beta.-SiC granules within relatively wide ranges of the composition of additives and the firing conditions, in which the abnormal grain growth is effectively suppressed by adding 0.05 to 5% by weight of .alpha.-SiC having a grain size as great as not less than 2 times that of .beta.-SiC in the B-C added .beta.-SiC.
However, since the composition range, the primary firing conditions and the HIP treating conditions are narrow in the case of the technique disclosed in Japanese Patent Application Laid-open No. 60-255,672, it is difficult to appropriately control the production conditions. Particularly when products having a great size are to be mass produced an an industrial scale, it is difficult to obtain uniform sintered bodies, and the strength is not so great, i.e., 70 kg/mm.sup.2, although the density is as high as 99.3% or more.
On the other hand, various production conditions are wide in the technique disclosed in Japanese Patent Application Laid-open No. 52-6,716, which has the composite structure consisting of the planar .alpha.-SiC grains having a greater aspect ratio, which is considered preferable from the standpoint of an increase in strength and toughness, and the finer .beta.-SiC grains. However, since the obtained product has a density lower than that of a product using .beta.-SiC powder alone, sintered bodies having high density, high strength, and high toughness, which have recently strongly been demanded, could not be attained. Further, since such products sintered at ordinary pressure have low density, and the pores present are of open pores, the products are unsuitable for densification by HIP treatment.